The present invention relates to a process for producing an alloy, the alloy and alloy containing sheets to which said alloy is applied. Particularly, the present invention provides a novel means to homogeneously alloy different kinds of metallic materials which are hardly soluble in each other and have melting points and densities widely different from each other, and relates to a useful means in particular when the alloy thus obtained is used in a powdery form. In addition, this means contributes to improve the recording property in the field of magnetically recording media.
In recent years, various alloys have been tried to be produced, but it have been difficult to obtain an purposed alloy when the metallic materials are hardly soluble in each other or have melting points and densities widely different from each other, due to occurrence of phase separation or segregation if the materials are simply melted together in a furnace.
Thus, first, a method to remove the influence of gravity at the time of solidification is considered, but it is too intricate on the process to realize a state of nongravitation, so that this process is unpractical.
Secondly, for a practical method, a powder metallurgical process has been proposed in which the powder of metallic material itself is tamped so as to utilize a solid phase reaction, but this yet have not been attained to the extent so as to be able to control in a homogeneous composition up to the finest area.
As other methods than these, means such as MA (mechanical alloying) have been known, but any product having higher properties has not been obtained for the long time required for alloying.
For example, a powder metallurgical process of prior art includes a step to preliminarily crush a metallic material into powder, a step to mix the obtained powder and then form molded pieces under pressure by a press, a step to make an alloy ingot by heating, and a step to crush this into fine powder. These steps hereinafter are described in the order.
(1) Crushing step of metallic material
Plural kinds of metallic materials are crushed under a non-oxidizing atmosphere into fine powder in order of 100 microns or less respectively. Materials apt to be oxidized or produce noxious powdery dust are hard to be handled. Besides, in the case of a metal which is soft and stretchable it is not easy to crush this material into powder.
(2) Molding step of the powder obtained above after mixing by press under pressure
Under a non-oxidizing atmosphere the powder of each metallic material is weighed in a prescribed quantity respectively. They are mixed and then formed into a molded product by means of a press. Metals which are easily oxidized must be formulated in excessive mole ratio. Therefore, some amount of unreacted materials are left in the final product, which often result in deterioration of properties.
(3) Alloy ingot making step by heating
The molded product is heated under a closely controlled atmosphere for long period to be alloyed.
(4) Crushing step of alloy ingot
The alloy ingot is taken out and in advance roughly crushed into powder of particle size between 100 and 500 microns. Then, the powder is finely ground into a powder of particle size about from 1 to 5 microns by wet milling using a ball mill, a planetary ball mill or the like, or by dry milling using a jet mill or the like to obtain an alloy powder.
In the above the process of prior art is outlined, which requires a very close control of atmosphere and very long time. Besides, when an alloy ingot is strongly crushed and milled into a fine powder of micron size, a broad distribution in the particle size of powder is brought.
For example, when the process is applied to the synthesis of magnetic recording element comprising an alloy powder mainly composed of Mn--Bi, the materials of Mn and Bi respectively are preliminarily crushed under a non-oxidizing atmosphere into fine powder in order of 100 microns or less, these are weighed in a prescribed quantity respectively, then are mixed and molded. The molded product further is heated under a non-oxidizing atmosphere for 10 to 20 days to form a MnBi ingot. Then, the ingot is submitted to wet milling to give an MnBi alloy powder. Such steps as mentioned above are employed. Accordingly, the process includes such drawbacks that it requires a long time for producing alloy powder and is inferior in productivity. Besides, due to the powerful crushing of ingot into fine powder in order of micron, deterioration of form distribution and change in quality of crystals often are brought. This causes the deterioration of magnetic property distribution of MnBi alloy powder as magnetic recording element and the variability of magnetic property of the final product originated from the nonuniform oxidation of material.
After all, by the powder metallurgical process, until now no means for producing alloy which can be controlled in a homogeneous composition up to the finest area could have been found, because there is a limit in providing material powder with smaller particle size, or it is impossible to employ the process when a metallic material is unstable.
Besides, when it was tried to obtain an alloy of fine particle by this powder metallurgical process, even if a powder of fine particle diameter was employed for alloying, it was impossible to control in a homogeneous composition up to the finest area, so that when the obtained alloy product was further crushed into powder, it could not be uniformly divided and no product having uniform particle size was obtained. This also brought a shortcoming to the process.